The present invention relates to weight rollers of the type used in centrifugal automatic transmissions or centrifugal clutches or the like found in motorbikes (broadly defined to include motor scooters), snow vehicles, propelled boats, or four-wheeled vehicles, or auxiliary mechanisms mounted on the above-mentioned vehicles such as superchargers, or agricultural machines, or general industrial machines.
Motorbikes or the like are provided with an automatic transmission which automatically changes the speed transmitted from a driving shaft to a driven shaft when the rotational speed of the engine changes. It operates by changing the width of a pulley groove, thus changing the radial contact position on the pulley of a power transmission belt (hereinafter simply referred to as transmission belt) such as an endless V-belt mounted around the pulley.
The mechanism for varying the groove width according to the rotational speed of the engine is arranged as illustrated in FIGS. 8 and 9 herein. The pulley 10 of which the groove width can be changed is comprised of a fixed plate 10a fastened to a rotary shaft 12 and a movable plate 10b movable axially of the rotary shaft 12. A guide plate surface 11 is formed on the rear side of the movable plate 10b and fixed to the rotary shaft 12. The gap between the guide plate surface 11 and the movable plate 10b gets narrower radially outwardly. Cylindrical weight rollers 1 are provided between the guide plate surface 11 and the movable plate 10b and are radially movable. The rollers 1 also rotate with the plates 10a and 10b. To restrain the weight rollers 1 from moving circumferentially relative to the plate 10b when they move radially on the pulley 10, the movable plate 10b (or guide plate surface) is provided with radial guides (or grooves) 10c (see FIG. 9), in each of which a weight roller 1 is arranged.
This mechanism works as follows: As the rotational speed of the rotary shaft 12 increases, the centrifugal forces acting on the weight rollers 1 will gradually increase, and the weight rollers 1 will shift on the rear surface 11 of the movable plate 10b in the direction of the centrifugal forces (or radially outward direction of the pulley) to shift the movable plate 10b axially towards the fixed plate 10a. As a result, the space between the movable plate 10b and the fixed plate 10a (which is the width of the pulley groove) will become narrower. Hence, in this arrangement, as the rotational speed of the rotary shaft increases, the endless transmission belt B mounted around the pulley will shift outwardly and contact the pulley nearer the outer circumference of the pulley. The effective diameter of the pulley 10 thus increases and the circumferential speed of the transmission belt on the pulley gradually increases. Thus, if the rotary shaft 12 is a driving shaft, the pulley size will change in such a way that the ratio of the speed of the driving shaft 12 and the transmitted speed of the driven shaft 13 increases automatically.
The weight rollers are required to have a high wear resistance since they will move (sliding rotation) radially continuously over the rear surface of the movable plate in response to any change in the rotational speed of the rotary shaft. To make this movement respond smoothly to any change in the rotational speed, the contacting circumferential surface of the weight rollers must have a small friction resistance.
When the mechanism is installed near an engine, the temperature of the weight rollers will rise to a considerable level, more specifically, to about 130.degree. C., due to the heat transferred from the engine and the frictional heat of the weight rollers themselves. Thus the weight rollers must also have an excellent heat resistance.
The weight rollers are also required to have an excellent fatigue resistance (load bearing capacity) since they must press the movable plate, opposing the transmitted force acting via the transmission belt, and they are exposed to repeated impactive oscillatory stresses from the engine.
To meet the aforementioned requirements, a variety of devices have been proposed up to the present. Typical inventions include the so-called wet type wherein grease is filled in the storage spaces for weight rollers (Japanese Patent Early Publication No. SHO-60-245864), and the so-called dry type wherein the surface of the weight rollers is covered with a material mixed with a self-lubricating reinforcing fiber to ease maintenance (Japanese Patent Early Publication No. SHO-60-245864).
In the former case, however, there are disadvantages such that, when the space is heated, the viscosity of the grease will drop, which will be scattered around by the centrifugal force, and that the grease deteriorates due to aging, and stable sliding properties and in turn speed changing properties cannot be maintained over a long period.
In the latter case (the dry type), the device is free of the aforementioned demerits. However, there is a disadvantage that, when the pulley is made by aluminum die casting for lighter weight and higher workability, the reinforcing fibers covering the weight rollers will damage the sliding surface on the rear of the movable aluminum plate, and as a result, stable speed changing properties cannot be assured. In particular, when molybdenum disulfide or graphite with a high specific weight is used as the lubricant, the contact parts such as the sliding surface tend to be damaged.
The aforementioned problems are also present for the weight rollers used in centrifugal clutches utilizing basically the same principle.
The weight rollers of the type wherein their surface is covered with a self-lubricating resin or the like have been produced mainly by injection-molding a resin in a mold in which a preheated metallic core has been placed beforehand, as disclosed in Japanese Patent Early Publications No. SHO-61-165058 and No. SHO-61-163821.
The weight rollers thus produced, however, generate internal stress in the resin portion thereof after production since the materials of the core and the resin portion are different and, in turn, have different thermal shrinkage rates. As a result, such weight rollers have a disadvantage that cracks are generated in the resin portion thereof within a relatively short time.
Weight rollers are required to be produced inexpensively since they are used frequently in mass-produced goods such as motorbikes. When weight rollers have been produced by the aforementioned methods, it has not been possible to reduce the production costs below a certain level because of the difficulty of automating the production thereof, etc.
The present invention has been made in view of the foregoing situation, and is intended to provide weight rollers, which are free of the aforementioned disadvantages, for automatic transmissions or centrifugal clutches, and are arranged in such a way that the production thereof can be made easily without generating internal stresses in the resin portion.